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SCIENCE 



[N. S. Vol. XXIX. No. 734 



omit more than it includes. For the Cali- 

 fornia district eleven are listed, within the 

 same period, the record being somewhat 

 Tagne, and possibly incomplete, for the 

 first half of the century. To these we may 

 tentatively add the Oregon or Klamath 

 earthquake of 1867 and the Sonora and 

 Arizona earthquake of 1887, raising the 

 number to thirteen. In other parts of the 

 United States were the New Madrid 

 (1811-12), the Charleston (1886), and a 

 relatively weak but probably destructive 

 shock in the New Madrid region in 1865. 

 The average interval between the in- 

 dividuals of the California series was nine 

 years, and the separate intervals, in order, 

 were: 12, 24, 3, 18, 8, 2, 1, 4, 15, 5, 6 and 

 8 years. As the centers of disturbance 

 were scattered through the whole district 

 and the areas of dangerous violence were 

 of moderate dimensions, the danger record 

 for any single locality was smaller, and the 

 intervals correspondingly larger. In San 

 Francisco, for instance, the last five de- 

 structive shocks have been separated by 

 intervals of 26, 3, 30 and 8 years. 



"While it is manifest at once that neither 

 of these sequences constitutes a rhythm, it 

 is quite conceivable that they represent in 

 some way a system of rhythms. They 

 might, for example, be composed of sev- 

 eral independent rhythms, each beating 

 with its own period ; or they might contain 

 imperfectly recorded rhythms, each re- 

 quiring for its interpretation some of the 

 less violent shocks not included in the de- 

 structive class. And if it were possible to 

 group the shocks according to place of 

 origin, it might be found that each earth- 

 quake center has its orderly law of se- 

 quence. But while the existence of such 

 a systematic arrangement seems within 

 the range of possibility, I regard it as 

 altogether outside the field of probability; 

 and I feel sure that any attempt to dis- 

 criminate rhjrthmic series on numerical 



grounds, without any other basis for 

 classification, would prove unprofitable. 



The single element of order which un- 

 questionably belongs to the sequence of 

 quakings is implied by the term after- 

 shock. Every great shock is followed by 

 a train of minor shocks, the length of the 

 train being roughly proportional to the 

 magnitude of the initial shock, and the 

 average strength and frequency of the 

 shocks diminishing with the lapse of time. 

 Usually also the great shock is preceded 

 by faint tremors, or by a few small shocks. 

 The prelude, the great shock and the train 

 of after-shocks, together constitute a typi- 

 cal seismic event, and if their sequence 

 could be absolutely depended on, the terror 

 of the great shock might rationally be pal- 

 liated by the thought that the worst is 

 over. But unfortunately there are ex- 

 ceptions, and the character of the ex- 

 ceptions is not reassuring. Occasionally 

 the prelude includes a shock of great 

 power, and occasionally the train of 

 after-shocks, instead of being wholly 

 subordinate in intensity, includes one 

 or more major shocks, rivaling the 

 initial shock in violence. Of the twenty- 

 five American examples fourteen were 

 normal and two abnormal, the others re- 

 maining unclassified because too little is 

 known of them. It is possibly significant 

 that the two abnormal earthquakes were of 

 exceptional power, the New Madrid head- 

 ing the list for the United States, and the 

 Takutat, of Alaska, being of the same 

 order of magnitude. The New Madrid 

 event began with a shock of great violence 

 at 2 o'clock on the morning of December 

 16, 1811, and this was followed by a long 

 series of vigorous after-shocks, among 

 which eight were noted as of special 

 strength and three were reported as equal- 

 ling or exceeding the initial shock. Of 

 the last-mentioned, one followed the initial 

 shock after an interval of five hours, and 



